The present invention relates to a polyacetal resin composition which is improved, when molded, in environmental stress cracking resistance (hereinafter referred to as ESCR) and in the physical properties of the weld portion and yet maintains the excellent moldability of polyacetal resin.
Polyacetal resins are highly balanced in mechanical strengths, impact resistance and sliding properties, are easy to process, and therefore, are widely used as a representative engineering plastic mainly in parts of electric or electronic appliances, automobile parts, and other mechanical parts. Many polyacetal resin molded articles are molded articles which are produced by insert molding or the like, and wherein strain may remain; or those parts which undergo stress continuously or repeatedly, such as the inside door handle of automobile, gear of an electronic appliance or the like. Most of these polyacetal resin molded articles are produced by multiple-gate molding and accordingly have a weld portion.
As polyacetal resins have come to be used in wider applications as above, the environment of their use have become diverse; therefore, it has become necessary for polyacetal resins to have improved performance. While ESCR referred to in the present invention is an important property for polyacetal resins, there has been no generally known method for improvement of the ESCR of polyacetal resin. Based on the literature on other resins, it is anticipated that the ESCR of polyacetal resin can be improved by allowing a polyacetal resin to have a higher molecular weight. However, allowing a polyacetal resin to have a higher molecular weight poses problems. For example, reduction in flowability, particularly in injection molding (e.g. reduced processability in multiple-cavity molding or thin material molding) may result.
Use of a metal salt of a fatty acid in polyacetal resin is described in JP-B-43-006101, JP-B-46-035980, JP-B-47-004943, JP-B-47-010531, JP-B-48-017378, JP-A-49-066747, JP-A-49-104940, JP-B-55-021778, JP-B-55-022508, JP-A-57-102943, JP-A-59-096157, JP-A-59-179653, JP-A-59-179654, JP-A-59-179655, JP-A-59-179656, JP-A-59-179657, JP-A-59-179658, JP-A-59-179659, JP-A-60-084348, JP-B-60-056748, JP-B-62-004422, JP-B-62-058387, JP-A-63-260949, JP-A-2-067352, JP-A-2-166151, JP-A-2-209944, JP-A-3-014857, JP-A-3-014859, JP-A-3-039351, JP-A-3-047819, JP-A-4-063857, JP-A-5-156118, JP-A-5-279550, JP-A-6-009854, JP-A-7-003117, JP-A-7-026111, JP-A-7-033953, JP-A-7-173368, JP-A-7-173369, JP-A-7-196889, JP-A-7-248162, JP-A-7-331028, WO 97/12937, Japanese Patent Publication No. 2677849, etc.
In the above literature, no mention is made of the use, in a polyacetal resin, of a difatty acid alkaline earth metal salt (referred to as component (B) in some places of the present specification) produced using two or more kinds of fatty acids and an alkaline earth metal. In the above-mentioned JP-A-2-067352, JP-A-2-166151 and Japanese Patent Publication No. 2677849, resin compositions comprising at least one kind of fatty acid alkaline earth metal salt are described, and the use of calcium stearate and calcium palmitate in combination is shown in their Examples. However, the difatty acid alkaline earth metal salts mentioned in the literature are the ones produced using only one kind of fatty acid, and it is not described in the literature that a difatty acid alkaline earth metal salt can be produced using two or more kinds of fatty acids.
In a series of JP-A-59-179653 to JP-A-59-179659, JP-B-62-004422 and JP-A-3-039351 mentioned above, a combination use of a difatty acid alkaline earth metal salt and a fatty acid is suggested; however, no Example of the combination use is given, and only the possibility of the combination use is mentioned. In fact, in the literature it is not described that the combination use of a particular difatty acid alkaline earth metal salt (B) and a particular fatty acid (C), which is employed in the present invention, can strikingly improve the ESCR and physical properties of weld portion of polyacetal resin molded article.
Many molded articles produced in multiple-gating have a weld portion. In such polyacetal resin molded articles, the physical properties of the weld portion are of importance. For instance, when a molded article having a weld portion has been subjected to aging at high temperatures, the physical properties of the weld portion are far inferior to those of the non-weld portion, and therefore, the physical property retention of the weld portion is important in designing a product. However, no effort has been made to improve the physical properties of the weld portion of a polyacetal resin molded article.
The heat aging resistance of polyacetal resin is generally improved by adding a fatty acid metal salt to a polyacetal resin as mentioned above. In the above-mentioned JP-B-55-022508, JP-A-2-209944 and JP-A-7-003117, for example, it is disclosed to add a fatty acid metal salt to a polyacetal resin to improve the heat aging resistance of the latter.
In this approach, the heat aging resistance of the non-weld portion is improved. However, the physical properties of the weld portion after aging are not fully satisfactory as compared with those of the non-weld portion.
The use of a fatty acid ester and fatty acid in polyacetal resin is disclosed. For example, polyacetal resin compositions obtained by adding, to a polyacetal resin, a fatty acid ester derived from a fatty acid and a polyhydric alcohol are disclosed in JP-B-53-031900, JP-A-60-104153, JP-A-2-263856, JP-B-5-018864, JP-B-5-035186, JP-A-6-107900, JP-A-6-293857, JP-A-7-018157 and Japanese Patent Publication No. 2522302.
Also, polyacetal resin compositions obtained by adding a hydroxyl group-containing fatty acid ester to a polyacetal resin are disclosed in JP-A-50-046756, JP-B-56-052940, JP-A-57-128740, JP-A-6-293856 and JP-A-7-011101.
Polyacetal resin compositions obtained by adding a fatty acid ester and a metal salt of a fatty acid to a polyacetal resin are disclosed in JP-B-63-002297, JP-A-4-063857, JP-A-7-173368, JP-A-7-196889 and JP-A-7-331028.
In any of the above approaches, however, the physical properties of the weld portion when the resulting polyacetal resin molded article has been tested for heat aging resistance, are not fully satisfactory as compared with the properties of the non-weld portion. Therefore, improvement on this point has been strongly needed.
In recent years, as the application of polyacetal resins have become wider, it has been found that polyacetal resins are deteriorated, in some cases, owing to acid rain or an acid generated in a small amount when a halogen group-containing resin is heated or the like.